Method and apparatus for dispensing material in a printer

ABSTRACT

A printer for printing a viscous material at predetermined positions forming a pattern on a substrate. The printer includes a frame, a device, mounted to the frame, having a number of perforations arranged to form the pattern, a support apparatus, coupled to the frame, that supports the substrate in a printing position beneath the device, and a material dispenser. The material dispenser has a chamber to contain the viscous material to be printed on the substrate. The chamber has an opening through which the viscous material is dispensed. The material dispenser is coupled to the frame, positioned over the device, and constructed and arranged to dispense the viscous material through the perforations in the device and onto the substrate. The material dispenser has a retraction device that prevents leakage of the viscous material from the opening after dispensing is complete.

RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 08,966,057, filed Nov. 7, 1997, now U.S. Pat. No. 5,947,022,which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an apparatus and process for dispensingmaterial, and more specifically to an apparatus and process fordispensing solder paste in a screen or stencil printer.

BACKGROUND OF THE INVENTION

In typical surface-mount circuit board manufacturing operations, astencil printer is used to print solder paste onto a circuit board.Typically, a circuit board having a pattern of pads or some other,usually conductive, surface onto which solder paste will be deposited isautomatically fed into the stencil printer and one or more small holesor marks on the circuit board, called fiducials, is used to properlyalign the circuit board with the stencil or screen of the stencilprinter prior to the printing of solder paste onto the circuit board. Insome prior art systems, an optical alignment system is used to align thecircuit board with the stencil. Examples of optical alignment systemsfor stencil printers are described in U.S. Pat. No. 5,060,063, issuedOct. 21, 1991 to Freeman, and in U.S. Pat. Re. No. 34,615, issued Jan.31, 1992, also to Freeman, each of which is incorporated herein byreference.

Once the circuit board has been properly aligned with the stencil in theprinter, the circuit board is raised to the stencil, solder paste isdispensed onto the stencil, and a wiper blade (or squeegee) traversesthe stencil to force the solder paste through apertures in the stenciland onto the board. As the squeegee is moved across the stencil, thesolder paste tends to roll in front of the blade, which desirably causesmixing and shearing of the solder paste so as to attain desiredviscosity to facilitate filling of the apertures in the screen orstencil. The solder paste is typically dispensed onto the stencil from astandard cartridge such as that manufactured by SEMCO Corporation.

In some prior art stencil printers, any excess solder paste remainingunder the squeegee after it has fully traversed the stencil, remains onthe stencil when the squeegee is returned to its initial position forprinting on a second circuit board. In some prior art screen printers, asecond squeegee is used that moves across the stencil in the directionopposite to that of the first squeegee. The first squeegee and thesecond squeegee are used on alternating boards to continually pass theroll of solder paste over the apertures of a stencil to print eachsuccessive circuit board. In the prior art stencil printers that utilizetwo squeegees, there is still the problem that at the end of amanufacturing day, or when the stencil is to be changed, excess solderpaste typically remains on the stencil and must be manually removed.Also, in these prior art printers, it is difficult to maintain adesirable viscosity because volatile solvents escape from the solderpaste thereby affecting the viscosity of the solder paste.

In the prior art stencil printers discussed above, the squeegee bladesare typically at a predetermined angle with the stencil to applydownward pressure on the solder paste to force the solder paste throughthe apertures in the stencil as the squeegee is moved across thestencil. The angle of the blade is selected based on the speed at whichthe blade traverses the stencil and based on the desired downwardpressure on the solder paste from the blade. It is desirable to maintaina consistent pressure on the solder paste as the squeegee traverses thestencil, however, in typical prior art printers, the pressure varies dueto variations in paste viscosity throughout a production run, and due tovariations in the angle of the squeegee caused by deformation of thesqueegee due to the pressure applied by the squeegee driving device.

It is desirable to provide a method and apparatus for dispensingmaterial onto a stencil of a printer that overcome the problemsdiscussed above.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide methods for dispensingmaterial and stencil printers having a dispensing apparatus thatovercome the problems of the prior art discussed above.

In a first embodiment of the present invention, a printer for printing aviscous material at predetermined positions forming a pattern on asubstrate is provided. The printer includes a frame, a device, mountedto the frame, having a number of perforations arranged to form thepattern, a support apparatus, coupled to the frame that supports thesubstrate in a printing position beneath the device, and a materialdispenser having a substantially cylindrical chamber to contain viscousmaterial to be printed on the device. The chamber has an opening throughwhich the viscous material is dispensed. The material dispenser iscoupled to a frame, positioned over the device, and constructed andarranged to dispense the viscous material through the perforations inthe device and onto the substrate.

Alternate versions of the first embodiment of the present inventioninclude a number of different features. In one version, the materialdispenser is constructed and arranged to be movable along a first axisacross the device while the viscous material is being dispensed from thechamber. In another version, the chamber has a cylindrical axisextending along a length of the chamber, and the interior surface of thechamber is coated with a coating material having a low coefficient offriction to allow mixing of the viscous material within the chamber whenthe material dispenser is moved across the device. In alternateembodiments, different coatings having both high and low coefficients offriction may be used on the interior surface of the chamber to enhancethe laminar flow of material in the chamber.

In another version of the first embodiment, the printer further includesa heater to heat the viscous material or a cooler to cool the material,and the material dispenser includes at least one port to receivepressurized paste to increase the paste pressure in the chamber to forceviscous material from the chamber.

In yet another version of the first embodiment, the material dispenserfurther includes a pressure sensor that senses pressure within thechamber, and the printer further includes a controller, coupled to thepressure sensor, that senses the pressure within the chamber andmaintains the pressure at a desired value.

In another version of the first embodiment, the material dispenser isadapted to receive a removable cartridge, and in some versions, theremovable cartridge is a standard SEMCO cartridge.

In still another version of the first embodiment, the material dispenserincludes a pair of inwardly facing blades with side dams that contactthe device during printing to prevent excess material from accumulatingon the device.

In another version of the first embodiment, the printer further includesa solder gathering squeegee arm that collects excess viscous materialremaining on the device when the material dispenser is lifted off of thedevice.

In a second embodiment of the present invention, a printer for printinga viscous material at predetermined positions forming a pattern on asubstrate is provided. The printer includes a frame, a device, mountedto the frame, having a number of perforations arranged to form apattern, a support apparatus that supports the substrate in a printingposition beneath the device, and a material dispenser having a chamberto contain the viscous material to be printed on the substrate. Thechamber has an opening through which the viscous material is dispensed.The material dispenser is positioned over the device, constructed andarranged to dispense the viscous material through the perforations inthe device and onto the substrate, and adapted to receive a removablecartridge that supplies the viscous material to the chamber. The chamberhas an inlet adapted to receive the viscous material from the removablecartridge.

Alternate versions of the second embodiment of the present invention mayinclude one or more of the features of versions of the first embodimentdiscussed above.

A third embodiment of the present invention provides a materialdispenser for dispensing a viscous material onto a stencil of a printer.The material dispenser includes a substantially cylindrical chamber tocontain the viscous material. The chamber has an opening through whichthe viscous material is dispensed. The material dispenser also includesa pair of inwardly facing blades and side dams mounted on the materialdispenser in close proximity to the opening. The blades are adapted tocontact the stencil during printing to prevent excess material fromremaining on the device.

In one version of the material dispenser, an interior surface of thechamber is coated with a coating material having a low coefficient offriction to allow mixing of the viscous material within the chamber whenthe material dispenser is moved across the device.

A fourth embodiment of the present invention provides a printer forprinting a viscous material at predetermined positions forming a patternon a substrate. The printer includes a frame, a device, mounted to theframe, having a number of perforations arranged to form the pattern, asupport apparatus, coupled to the frame, that supports the substrate ina printing position beneath the device and a material dispenser having achamber to contain the viscous material to be printed on the substrate.The chamber has an opening through which the viscous material isdispensed. The material dispenser is coupled to the frame, positionedover the device, and constructed and arranged to dispense the viscousmaterial through the perforations in the device and onto the substrate.The material dispenser has a retraction device that prevents leakage ofthe viscous material from the opening after dispensing is complete.

In one version, the retraction device includes at least one plunger thatis extendable into the chamber to vary the volume of the chamber, andthe retraction device can include an actuator coupled to the plunger tocontrol movement of the plunger. The actuator may have a first air inletand a second inlet, and the actuator can be constructed and arranged toprovide movement of the plunger in a first direction upon theapplication of pressurized air at the first inlet and to providemovement of the plunger in a second direction upon the application ofpressurized air at the second inlet.

In versions of the fourth embodiment, the material dispenser can includeat least one port to receive pressurized air to provide air pressure tothe chamber to force the viscous material from the chamber duringdispensing, and the material dispenser can include a pressure sensor tosense pressure within the chamber, and the printer can further include acontroller coupled to the pressure sensor that senses the pressurewithin the chamber and controls the pressurized air provided through theport to maintain the pressure within the chamber at a desired value. Theprinter can be adapted to receive a removable cartridge that suppliesthe viscous material to the chamber, and the chamber can have an inletto receive the viscous material from the removable cartridge. Thepressurized air can be provided to the cartridge to force the viscousmaterial from the cartridge into the chamber.

In a fifth embodiment, a printer is provided for printing a viscousmaterial at predetermined positions forming a pattern on a substrate.The printer includes a frame, a device, mounted to the frame, having anumber of perforations arranged to form the pattern, a supportapparatus, coupled to the frame, that supports the substrate in aprinting position beneath the device, and a material dispenser having achamber to contain the viscous material to be printed on the substrate.The chamber has an opening through which the viscous material isdispensed, and the material dispenser is coupled to the frame,positioned over the device, and constructed and arranged to dispense theviscous material through the perforations in the device and onto thesubstrate. The printer also includes means for retaining the viscousmaterial in the chamber after dispensing is complete.

In versions of the fifth embodiment, the means for retaining includemeans for increasing a volume of the chamber, and the material dispenseris adapted to receive a removable cartridge that supplies the viscousmaterial to the chamber, the chamber having an inlet to receive theviscous material from the removable cartridge. The printer can furtherinclude means for applying pressure to the viscous material in thecartridge to force viscous material into the chamber from the cartridgeand to force viscous material from the chamber through the opening.

In a sixth embodiment of the present invention, a method is provided forprinting a pattern of viscous material on a substrate using a dispenserhaving a chamber with an inlet to receive viscous material and an outletfrom which viscous material is dispensed. The method includes steps ofpositioning the dispenser over the substrate, applying pressure to theviscous material in the chamber to dispense viscous material from theoutlet of the chamber onto the substrate, relieving the pressure appliedto the viscous material to discontinue dispensing, and expanding thevolume of the chamber to retain the viscous material in the chamber.

The method of the sixth embodiment can further include steps ofdecreasing the volume of the chamber prior to dispensing material ontothe substrate, and coupling a removable cartridge containing the viscousmaterial to the dispenser such that an outlet of the removable cartridgeis coupled to the inlet of the chamber. In versions of the sixthembodiment, in the step of applying pressure, air pressure is suppliedto the removable cartridge.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the drawings which are incorporated herein by reference and in which:

FIG. 1 provides a front view of a stencil printer in accordance with oneembodiment of the present invention;

FIG. 2 provides a perspective view of a material dispensing head used inthe stencil printer of FIG. 1;

FIG. 3 provides a cross-sectional side view of the material dispensinghead shown in FIG. 2;

FIG. 4 shows a close up side view of a chamber of the material dispenserof FIG. 2;

FIG. 5 shows a bottom view of the material dispenser in greater detail;

FIG. 6 shows a side view of the material dispenser with a soldergathering arm;

FIG. 7 shows a front view of another embodiment of a dispensing headused in the stencil printer of FIG. 1;

FIG. 8 shows a top view of the dispensing head of FIG. 7;

FIG. 9 shows a side view of the dispensing head of FIG. 7;

FIG. 10 shows a cross-sectional side view of the dispensing head takenalong line 10-10 of FIG. 8;

FIG. 11 shows a cross-sectional view taken along line 11—11 of FIG. 10;

FIG. 12 shows a side view of a retraction assembly used in thedispensing head of FIG. 7;

FIG. 13 shows a top view of the retraction assembly of FIG. 12; and

FIG. 14 shows a perspective view of the housing of the dispensing headof FIG. 7 with the retraction assembly removed.

DETAILED DESCRIPTION

For purposes of illustration, embodiments of the present invention willnow be described with reference to a stencil printer used to printsolder paste onto a circuit board. One skilled in the art willappreciate, however, that embodiments of the present invention are notlimited to stencil printers that print solder paste onto circuit boards,but rather, may be used in other applications requiring dispensing ofother viscous materials such as glues and encapsulents. Further, stencilprinters in accordance with embodiments of the present invention are notlimited to those that print solder paste on circuit boards, but rather,include those used for printing other materials on a variety ofsubstrates. Also, the terms screen and stencil may be usedinterchangeably herein to describe a device in a printer that defines apattern to be printed onto a substrate.

FIG. 1 shows a front view of a stencil printer 10 in accordance with oneembodiment of the present invention. The stencil printer 10 includes aframe 12 that supports components of the stencil printer including acontroller 14, a stencil 16, and a dispensing head 100 having adispensing slot 102 from which solder paste may be dispensed.

The dispensing head 100 is coupled to a first plate 18 using twothumbscrews 22. The first plate 18 is coupled to a second plate 20 whichis coupled to the frame 12 of the stencil printer 10. The first plate 18is coupled to the second plate 20 in such a manner that the first platecan be moved with respect to the second plate along a z axis, the z axisbeing defined by the coordinate axis system 23 shown in FIG. 1. Thefirst plate is moved by motors under the control of the controller 14.

The second plate 20 is movably coupled to the frame 12 such that thesecond plate 20 can move with respect to the frame 12 along an x axis,the x axis also being defined by the coordinate axis system 23. Asdescribed below in further detail, the movements of the first and secondplates allow the dispensing head 100 to be placed over the stencil 16and moved across the stencil to allow printing of solder paste onto acircuit board.

Stencil printer 10 also includes a conveyor system having rails 24 fortransporting a circuit board 26 to a printing position in the stencilprinter. The stencil printer has a number of pins 28, positioned beneaththe circuit board when the circuit board is in the dispensing position.The pins are used to raise the circuit board 26 off of the rails 24 toplace the circuit board in contact with, or in close proximity to, thestencil 16 when printing is to occur.

The dispensing head 100 is configured to receive two standard SEMCOthree ounce or six ounce solder paste cartridges 104 that provide solderpaste to the dispensing head during a printing operation. Each of thesolder paste cartridges 104 is coupled to one end of a pneumatic airhose 30. As readily understood by those skilled in the art, thedispensing head could be adapted to receive other standard, ornon-standard, cartridges. The other end of each of the pneumatic airhoses is attached to a compressor that under the control of thecontroller 14 provides pressurized air to the cartridges to force solderpaste to flow from the cartridges into the dispense head 100 and ontothe screen 16. Mechanical devices, such as a piston, may be used inaddition to, or in place of, air pressure to force the solder paste fromthe SEMCO cartridges into the dispensing head.

In one embodiment of the present invention, the controller 14 isimplemented using a personal computer using the Microsoft DOS orWindows® NT operating system with application specific software tocontrol the operation of the stencil printer as described herein.

The stencil printer 10 operates as follows. A circuit board 26 is loadedinto the stencil printer using the conveyor rails 24. The dispensinghead is then lowered in the z direction until it is in contact with thestencil 16. Pressurized air is provided to the cartridges 104 while thedispensing head is moved in the x direction across the stencil 16. Thepressurized air forces solder paste out the cartridges and createspressure on the solder paste in the dispensing head forcing solder pastefrom the dispensing slot of the dispensing head through apertures in thestencil 16 and onto the circuit board 26. Once the dispensing head 100has fully traversed the stencil 16, the circuit board is lowered backonto the conveyor rails 24 and transported from the printer so that asecond circuit board may be loaded into the printer. To print on thesecond circuit board, the dispensing head is moved across the stencil inthe direction opposite to that used for the first circuit board.Alternatively, a squeegee arm (as described below) could swing in tocontain the solder paste in the dispenser, and the dispenser can then belifted in the z direction and moved back to its original position toprepare to print on the second circuit board using a similar directionstroke. Further description of the operation and construction of thedispensing head 100 are provided below with reference to FIGS. 2-5.

The dispensing head 100 includes a housing 105 coupled to two supportingarms 106. At one end of each of the supporting arms 106 are thumbscrews22 that provide for easy removal and installation of the dispensing head100. The housing 105 has two ports 108 that are adapted to receive astandard SEMCO solder paste cartridge. In the illustrative embodimentdescribed herein, the dispensing head is adapted to receive two SEMCOcartridges, however, the dispensing head may be adapted to include moreor less cartridges than the two shown herein. The number of cartridgesused is selected based on the length L of the dispensing head and thecapacity of the cartridge used. The length L is determined in part basedon the width of the circuit boards to be printed upon. If the size ofthe circuit board changes, then the dispensing head may be replaced by anew dispensing head having a length L sized for the new circuit boards.The effective length of the slot 102 may also be reduced to accommodatesmaller circuit boards by partially covering a portion of the slot.

The housing 105 includes a cylindrical chamber 110 in which solder pastereceived from the SEMCO cartridges is stored prior to being dispensedonto the stencil. In one embodiment, the interior walls of the chamber110 are coated with a material having a low coefficient of friction.Other materials, having a wide range of coefficients of friction, areused for the coating to create a desired laminar flow of solder paste inthe chamber. In one embodiment, a number of different coatings having avariety of coefficients of friction may be used to create the desiredlaminar flow. At the slot 102, the housing has two inwardly facingblades 112 and 114. Each of the inwardly facing blades has a lengthapproximately equal to the length L of the slot, a width equal toapproximately 0.38 inches, and a thickness of approximately 0.004 to0.010 inches. Each of the blades 112 and 114, in one embodiment of thepresent invention, is made from spring steel. At each end of the bladesis a side dam 103. While other materials such as plastic could be usedto make the blades, the use of spring steel provides long life withcontinued resiliency. Each of the blades is arranged to provide an angleof approximately 15 degrees between the blade and the top surface of thestencil. Depending in part on the material used to make the blades andthe speed at which the material dispenser traverses the stencil, theblades could be oriented at angles other than 15 degrees.

The dispensing head 100, in one embodiment of the present invention,further includes a pressure sensor 116 and a temperature controller 120.Incorporated within the temperature controller is a thermoelectricdevice and a temperature sensor. The pressure sensor 116 and thetemperature controller 120 are coupled to the controller 14 throughcables 118 and 122 respectively. The controller 14 is configured tomeasure the pressure and the temperature of the solder paste within thechamber 110 based on signals received from the sensors, and thecontroller controls the pressure and temperature by adjusting the forceapplied to the solder paste in the cartridges and by controlling thethermoelectric device contained in the temperature controller. In oneembodiment, the pressure of the solder paste is maintained in thechamber by applying pressurized air at approximately 1-25 pounds persquare inch to each of the cartridges 104. The temperature of the solderpaste in the chamber is maintained during the printing operation atapproximately 68 degrees Fahrenheit. In one embodiment, the pressuresensor is implemented using a pressure transducer with a programmableregulator and the temperature controller is implemented using atemperature probe with a thermoelectric device that can provide coolingas well as heating of the solder paste.

As discussed above, when the dispensing head is in the lowered printingposition so that it is in contact with the stencil, the stencil printer10 operates by forcing solder paste from the dispensing head 100 ontothe stencil using air pressure applied to each of the SEMCO cartridgesas the dispensing head moves across the stencil. In the printingposition, the blades 112 and 114 contact the top surface of the stencil.For each direction that the dispensing head moves across the stencil,one of the blades 112 and 114 will be a trailing blade and will scrapeany excess solder paste off the stencil. For example, when thedispensing head 100 moves in the direction of arrow 122 in FIG. 3, blade112 will be the trailing blade removing any excess solder paste from thestencil. The orientation of the blades and the use of a cylindricalchamber in the dispensing head causes the solder paste within thechamber 110 to roll or flow in the direction of arrow 124 when thedispensing head is moved in the direction of arrow 122. The movement ofthe solder paste within the chamber causes mixing and shearing of thesolder paste which helps to maintain the desired temperature andviscosity of the solder paste. The coating for the internal walls of thechamber 110 allows the solder paste to move freely within the chamber.

At the conclusion of printing, when it is desired to lift the dispensinghead off of the stencil, the controller 14 turns off the pressurized airsource, prior to lifting the dispensing head. This causes the solderpaste to remain in the chamber 110 when the dispensing head is liftedand effectively reduces the amount of solder paste that is left on thestencil when printing is complete.

To further reduce the amount of residual solder paste on the stencil, asolder gathering squeegee arm may be used to remove excess solder paste.One example of a solder gathering squeegee arm that could be used inembodiments of the present invention is described in U.S. Pat. No.5,044,306, which is incorporated herein by reference. FIG. 6 shows asolder gathering arm 130 incorporated into the dispensing head 100. Thesolder gathering arm includes a squeegee 131. The solder gathering armis mounted to a bracket 132 which in turn is mounted to one or both ofthe supporting arms 106. The solder gathering arm is mounted to thebracket 132 using a rotatable joint 134, and the bracket 132 is movablewith respect to the supporting arms 106 in the direction shown by arrow122.

The solder gathering arm operates as follows. When the dispensing head100 is lifted slightly off of the stencil, the bracket 132 is moved inthe direction of arrow 122, and the solder gathering arm is rotated inthe direction shown by arrow 136 causing the squeegee 131 to scrapealong the stencil and remove any excess solder on the stencil. Therotation of the solder gathering arm and the movement of the bracket 132is accomplished using motors or actuators under the control of thecontroller 14.

In embodiments of the present invention, instead of using the soldergathering arm, the blades 112 and 114 may be attached to motors oractuators that under the control of the controller cause the blades tomove towards each other, scraping any excess solder paste from thescreen and completely closing the slot 102. Alternatively, a pair ofmovable blades, in addition to blades 112 and 114, may be mounted to thedispensing head and be movable towards each other to scrape excesssolder paste from the stencil.

In another embodiment of the present invention, which will now bedescribed with reference to FIGS. 7-13, pressure is reduced within thechamber of a dispensing head to draw excess solder paste into thechamber at the conclusion of printing. FIG. 7 shows a side view of adispensing head 200 in accordance with this embodiment of the invention.The dispensing head 200 is similar to dispensing head 100 and is mountedto the printer using two thumb screws 222 coupled to two supporting arms206. The dispensing head 200, like dispensing head 100, is designed toaccommodate two standard solder paste cartridges 104, which are mountedto ports 208. The dispensing head includes a bracket 224 to whichconductive proximity switches 225 are mounted. The conductive proximityswitches detect when the solder paste level in the cartridges dropsbelow a predetermined level, and at that time, further printing isprevented and an indication is provided to an operator of a low solderpaste condition. In one embodiment, the conductive proximity switchesare implemented using switches capable of detecting a conductivematerial, such as those available from Omron Corporation under part no.E2K-F 10MC2. In other embodiments, more or less solder paste cartridgesmay be used.

The dispensing head 200 has a housing 205 containing a cylindricalchamber 210. The housing has two ports 208 for passing solder paste fromthe cartridges 104 into the cylindrical chamber 210. Similar todispensing head 100, dispensing head 200 has blades 212 and 214 and sidedams 203. In one embodiment, the side dams have a lower portion and anupper portion. The lower portion is formed from a relatively rigidmaterial (such as urethane having a durometer of 70 shore D) to preventwear, while the upper portion is made from a more compliant material(such as urethane having a durometer of 10-15 shore A) to allow slightcompression of the side dams as the dispensing head is moved across thestencil. In addition, the dispensing head 200 has a pressure sensor 216to detect pressure in the chamber 210 and a temperature sensor to detecttemperature of the solder paste in the chamber. Within the housing, asbest seen in FIG. 11, adjacent to the chamber are channels 209, 211 and213. Channels 209 and 211 arc used to more evenly distribute solderpaste from the cartridges through the ports 208 to the chamber andchannel 213 enables the pressure sensor 216 to measure pressure withinthe chamber.

Dispensing head 200 differs from dispensing head 100 in that it includesa solder paste retraction assembly 230. As described below in greaterdetail, after the completion of printing on a substrate, the solderpaste retraction assembly causes the effective volume of the chamber 210to increase to create a negative pressure within the chamber whichcauses excess solder paste to be retracted into the chamber.

The retraction assembly 230 includes four actuators 232 having pistons238 coupled to two plunger mounting brackets 244. Each of the plungermounting brackets 244 is coupled to two elongated, unshaped plungers 240using screws 241. Each of the actuators is mounted to an actuatormounting bracket 242 which is mounting to the housing 205 using screws243.

As best seen in FIG. 14, which shows the housing 205 with the retractionassembly and cartridges removed, the housing 205 has an upper channel248, two intermediate channels 250 and four lower slots 252. The upperchannel extends across the housing and is sized to accommodate theactuator bracket 242. The two intermediate channels 250 are formedwithin the actuator channel and are sized to accommodate the plungermounting brackets 244. Two of the four lower slots 252 are formed ineach of the intermediate channels 250. Each of the lower slots is sizedto accommodate one of the plungers 240.

In one embodiment of the present invention the actuators are implementedusing double acting, pancake air cylinders having a {fraction (9/16)}inch diameter and a ⅛ inch stroke, available from Bimba of Monee, ILunder part no. FOD-020.125-H. These actuators have two pressurized airinlet ports 234 and 236 that in embodiments of the present invention arecoupled to controllable sources of pressurized air to control movementof the pistons 238. Pressurized air is provided to inlet ports 234 ofthe actuators to extend the pistons, and pressurized air is provided toinlet ports 236 to retract the pistons.

In one embodiment, the u-shaped plungers are made from a resilientmaterial such as urethane having a durometer of 65-70 shore D. In otherembodiments other materials may be used for the plungers. The plungersprovide a seal in the elongated slots 252 to prevent solder paste in thechamber from escaping through the slots 252.

The operation of the dispensing head 200 will now be described. Thedispensing head 200 operates during a printing cycle in substantiallythe same manner as dispensing head 100 to print a pattern of solderpaste, or some other substance, onto a substrate, such as a circuitboard, using a stencil having the pattern defined thereon. At theconclusion of printing, pressurized air is provided to inlet ports 236to retract each of the pistons 238 into the actuators and cause each ofthe plungers 240 to be raised in the elongated slots 252. When theplungers are raised, the effective volume of the chamber 210 increasescreating a momentary vacuum condition in the chamber that draws thesolder paste into the chamber. Prior to the start of printing on anothersubstrate, pressurized air is provided to inlet ports 234 to extend eachof the pistons from the actuators and cause the plungers to be loweredin the elongated slots 252.

In one embodiment of the present invention used for printing solderpaste, the pistons and the plungers are moved a distance ofapproximately 0.125 inches between the extended and retracted positions.In this embodiment, the volume of the chamber increases by approximately2% to 4% when the pistons are retracted. In other embodiments, dependingon characteristics of the material being printed, and depending on thedesign of the chamber, it may be desirable to change the volume of thechamber by other values.

In the illustrative embodiment of the dispensing head 200 describedabove, four plungers with four actuators are used in the retractionassembly, and the housing includes two intermediate channels and fourlower slots to accommodate the retraction assembly. In otherembodiments, it may be desirable to use more or less plungers,actuators, intermediate channels and lower slots depending on thedimensions of the printing head and chamber. In addition, in otherembodiments of the present invention, devices other than plungers may beused to expand the volume of the chamber.

In the illustrative embodiment described above, air actuators are usedto extend and retract the pistons. As understood by those skilled in theart, in other embodiments, other mechanisms, such as electrical motors,could be used to move the pistons.

In embodiments of the present invention that do not include thedispensing head 200 with the retraction assembly, it has been found thatsolder paste can leak from the chamber after printing has occurred whenthe substrate is removed from the stencil. Solder paste that leaks fromthe chamber is at times able to flow between the stencil and the bladesand undesirably collect on the back sides of the blades. The use of theretraction assembly to draw solder paste into the chamber after printingis effective in reducing the leaking of solder paste from the chamber.

U.S. patent application Ser. No. 08/598,288, filed on Feb. 8, 1996,assigned in part to the assignee of the present application, andincorporated herein by reference, discloses a screen printer having asolder applicator that applies solder paste to a stencil. In oneembodiment, the solder applicator includes a rotatable member that mixessolder paste in the solder applicator. Embodiments of the presentinvention may include a rotatable member within the chamber to assist inthe mixing of solder paste in the chamber.

Embodiments of the present invention described above may also beincorporated in a dual track stencil printer such as those described inU.S. patent application Ser. No. 08/802,934, filed Feb. 21, 1997, whichis incorporated herein by reference.

Embodiments of the present invention described above include standardSEMCO cartridges for loading solder paste into the dispensing head.Other removable cartridges may be used in place of the SEMCO cartridges.However, it is desirable to use a standard replaceable cartridge.

The chamber of dispensing heads in embodiments of the present inventionhave been described as being cylindrical. The dispensing head need notbe cylindrical, but rather, other shapes having a substantially circularcross-section to allow rolling of the solder paste within the chamber tocreate a substantially laminar flow may also be used. In addition, inone embodiment, the chamber is generally kidney-shaped, and the specificshape is programmable for controlling the flow of solder paste withinthe chamber based on the specific solder paste material used, the speedof the dispensing head across the stencil and based on any otherfactors.

The dispensing head of embodiments of the present invention is describedas being used with a stencil printer. Stencil printers such as theUltraprint 3000 stencil printer and the AP Series stencil printer, bothavailable from MPM Corporation of Franklin, Mass. can readily be adaptedfor use with embodiments of the present invention.

Embodiments of the present invention overcome problems associated withprior art stencil printers by providing dispensing heads and stencilprinters that effectively remove excess solder paste from the stencil.In addition, the pressure applied to the solder paste to force itthrough apertures in the stencil is precisely controlled using a closedloop feedback system.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications and improvements willreadily occur to those skilled in the art. Such alterations,modifications and improvements are intended to be within the scope andspirit of the invention. Accordingly, the foregoing description is byway of example only and is not intended as limiting. The invention'slimit is defined only in the following claims and the equivalentsthereto.

What is claimed is:
 1. A printer for printing a viscous material atpredetermined positions forming a pattern on a substrate, the printercomprising: a frame; a device, mounted to the frame, having a number ofperforations arranged to form the pattern; a support apparatus, coupledto the frame, that supports the substrate in a printing position beneaththe device; and a material dispenser having a ridged chamber to containthe viscous material to be printed on the substrate, the chamber havingan opening through which the viscous material is dispensed, the materialdispenser being coupled to the frame, positioned over the device, andconstructed and arranged to dispense the viscous material through theperforations in the device and onto the substrate, the materialdispenser having a retracttion device, extendable into the chamber, thatprevents leakage of the viscous material form the opening afterdispensing is complete, the retraction device being configured andarranged to increase the volume of the rigid chamber after dispensing,the material dispenser further having at least one port and a removablecartridge coupled to the port, the removable cartridge containingmaterial to be dispensed and having an inlet to receive pressurized airto force the material from the cartridge through the port and out of theopening of the chamber.
 2. The printer of claim 1, wherein theretraction device includes at least one plunger that is extendable intothe chamber to vary a volume of the rigid chamber.
 3. The printer ofclaim 2, wherein the retraction device includes an actuator coupled tothe plunger to control movement of the plunger.
 4. The printer of claim3, wherein the actuator has a first air inlet and a second inlet, andwherein the actuator is constructed and arranged to provide movement ofthe plunger in a first direction upon the application of pressurized airat the first inlet and to provide movement of the plunger in a seconddirection upon the application of pressurized air at the second inlet.5. The printer of claim 4, wherein the material dispenser is constructedand arranged to be movable along a first axis across the device whilethe viscous material is being dispensed from the chamber.
 6. The printerof claim 5, wherein the chamber has a cylindrical axis extending along alength of the chamber, and wherein the cylindrical axis is orthogonal tothe first axis.
 7. The printer of claim 6, wherein the materialdispenser includes a pressure sensor to sense pressure within thechamber, and wherein the printer further comprises a controller coupledto the pressure sensor that senses the pressure within the chamber andcontrols the pressurized air provided through the port to maintain thepressure within the chamber at a desire value.
 8. The printer of claim7, wherein the viscous material is solder paste and the device is astencil for printing solder paste on a circuit board.
 9. The printer ofclaim 1, wherein the retraction device includes a plurality of plungersdisposed along a portion of the chamber.
 10. A printer for printing aviscous material at predetermined positions forming a pattern on asubstrate, the printer comprising: a frame; a device, mounted to theframe, having a number of perforations arranged to form the pattern; asupport apparatus, coupled to the frame, that supports the substrate ina printing position beneath the device; a material dispenser having arigid chamber to contain the viscous material to be printed on thesubstrate, the chamber having an opening through which the viscousmaterial is dispensed, the material dispenser being coupled to theframe, positioned over the device, and constructed and arranged todispense the viscous material through the perforations in the device andonto the substrate; means for retaining the viscous material in thechamber after dispensing is complete including means for increasing avolume of the rigid chamber, a cartridge coupled to the chamber thatprovides material to the chamber; and means for applying pressure to theviscous material in the cartridge to force material from the cartridgeinto the chamber and out of the opening of the chamber.
 11. The printerof claim 10, wherein the means for retaining includes a plurality ofplungers disposed within the chamber.
 12. A method for printing apattern of viscous material on a substrate using a dispenser having arigid chamber with an inlet to receive visous material and an outletfrom which viscous material is dispensed, the chamber having a volume,the method comprising steps of: positioning the dispenser over thesubstrate; extending a plunger into the chamber to contact the materialin the chamber and decrease the effective volume of the chamber;applying pressure to the viscous material in the chamber to dispenseviscous material form the outlet of the chamber onto the substrate;relieving the pressure applied to the viscous material to discontinuedispensing; expanding the volume of the chamber to retain the viscousmaterial in the chamber by at least partially retracting the plungerform the chamber; coupled a removable cartridge containing the viscousmaterial in the chamber by at an outlet of the removable cartridge iscoupled to the inlet of the chamber.
 13. The method of claim 12, whereinin the step of applying pressure, air pressure is supplied to theremovable cartridge.
 14. A system for depositing a viscous material atpredetermined position forming a pattern on a substrate, the systemcomprising: a frame; a device, mounted to the frame, having a number ofperforations arranged to form the pattern; a support apparatus, coupledto the frame, that supports the substrate in a printing position beneaththe device; and a material dispenser coupled to the frame and including:a chamber to contain the viscous material to be printed on thesubstrate, the chamber having an opening through which the viscousmaterial is dispensed onto the substrate; a retraction device,extendable into the chamber, that prevents leakage of the viscousmaterial from the opening after dispensing is complete, the retractiondevice increasing the volume of the chamber after dispensing; acartridge coupled to the chamber and containing material to bedispensed, the cartridge having an inlet to receive pressurized air toforce the material from the cartridge into the chamber and out of theopening of the chamber.
 15. The system of claim 14, wherein theretraction device includes a plurality of plungers that are extendableinto the chamber to vary the volume of the chamber.
 16. The system ofclaim 15, wherein the plurality of plungers are disposed along a portionof the chamber opposite the opening.
 17. The system of claim 16, whereinthe material dispenser is movable across the device in a direction alonga first axis, and wherein the chamber is substantially cylindricalhaving a cylindrical axis that is perpendicular to the first axis. 18.The system of claim 17, wherein the plurality of plungers are disposedalong an axis that is parallel to the cylindrical axis.